The goal of this work is the elucidation of the mechanisms controlling the rate of protein synthesis at the cytoplasmic lev%l in mammalian cells. The tissue utilized in the present studies is the Ehrlich ascites tumor cell growing in suspension culture. This cell demonstrates cytoplasmic control mechanisms which respond vigorously to alterations in nutrient (essential amino acid, glucose, or serum) concentration in the medium. We have recently demonstrated that upon nutrient deprival one such mechanism operates to inhibit polypeptide chain initiation by inhibiting the formation of the initiation complex consisting of the native 40s ribosomal subunit plus bound Met-tRNAf, the initiating aminoacyl tRNA. Formation of this complex requires the Met-tRNAf Binding Factor, GTP, Met-tRNAf, and an active native 40s subunit. We are examining these components to determine which is modified during nutrient deprival. Current studies aRe focused upon an inhibitor of this reaction which is present in Ehrlich cells and which appears, like the hemin-controlled reticulocyte inhibitor, to phosphorylate the smaller subunit of the Met-tRNAf Binding Factor.